1. Field of the Invention
The present invention relates to a method of preparing an oxide superconducting thin film by a vapor phase method such as laser ablation, for example.
2. Description of the Background Art
In a typical example of a product which is prepared from an oxide superconductive material, such as an oxide superconducting wire, for example, an oxide superconducting thin film is formed on a suitable substrate, so that the oxide superconductive material is held by the substrate and shaped into a desired configuration.
In order to form such an oxide superconducting thin film on a substrate as described above, a vapor phase method, for example, is employed. Among various vapor phase methods, laser ablation has been watched with interest particularly in recent years, due to its capability of forming an oxide superconducting thin film under a low temperature at a high speed.
FIG. 3 illustrates a crystal lattice of an oxide superconducting thin film 2, which is formed on a substrate 1 by a vapor phase method. As shown in FIG. 3, it is relatively easy to perpendicularly orient the c-axes in the crystal lattice of the oxide superconducting thin film 2 with respect to the substrate 1. Such orientation of the c-axes is easily attained particularly by laser ablation.
On the other hand, FIGS. 4 and 5 are top plan views illustrating crystal lattices of the oxide superconducting thin film 2 shown in FIG. 3.
As to such an oxide superconducting thin film 2, it is known that current flows in a-b planes, which are parallel to the surface of the substrate 1. In order to enable feeding of large current in the oxide superconducting thin film 2, therefore, the directions of the a-axes and the b-axes may be regulated as shown in FIG. 4. In general, such a-axes and b-axes are arbitrarily inclined as shown in FIG. 5.
In relation to such an oxide superconducting thin film as shown in FIG. 5, it has been recognized that critical current density J.sub.c in a zero field as well as critical current density J.sub.c -B under a magnetic field are reduced as inclination 3 between the a-axes of adjacent crystal lattices is increased in a range of 0.degree. to 45.degree..
When the substrate 1 is formed of a polycrystalline material, the directions of the a-axes and the b-axes are easily irregularlized as compared with those of a single-crystalline substrate. In relation to this, it is necessary to use a long substrate having proper flexibility, in order to obtain an oxide superconducting wire by forming an oxide superconducting thin film on such a substrate. In general, it is difficult to provide a long flexible substrate by a single-crystalline material. In order to obtain a long flexible substrate, a polycrystalline material must be employed in general.